Be sure that the fuel control lever on the injection pump goes to the stop in full throttle.
At 80 you should be into a number of pounds of boost. For proper fuel enrichment to take place the boost signal must get to the aneroid on top of the injection pump. To protect the motor from overboosting there is a switchover valve in the signal hose from the intake to the aneroid. The valve often plugs with soot and can be blown out. Its normal unenergized state is to pass the boost signal through.
Check the boost signal by teeing into the line from the valve to the aneroid. The boost should reach 9-10psi by 2500rpm (my estimated figure) under full throttle. The pressure should drop also as soon as the throttle is released.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician